Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 AM EST Fri Jan 27 2023

Valid 12Z Fri Jan 27 2023 - 12Z Mon Jan 30 2023

...Northern Rockies and Intermountain West...
Days 1-3...

A deep and sprawling low vortex will remain centered just north of
Hudson Bay through the middle of next week. Surrounding troughing
currently spans the CONUS from the Pacific Northwest to the
Eastern Seaboard. An amplifying shortwave trough currently
shifting from northern Alaska through the Yukon will dive south
with Arctic air over BC through tonight, becoming positively
tilted over the Pacific Northwest Saturday then down California
Sunday/Monday. A leading cold front will become banked into the
eastern side of the terrain of the Northern Rockies today through
Saturday morning when it is shoved south by the reinforcing
trough.

As the cold front banks into the terrain today enhanced fgen from
lee-side cyclogenesis shifting south over the northern High Plains
and a strong surface ridge shifting down the lee of the Canadian
Rockies will combine with strengthening upslope flow on increasing
NE winds to bring waves of heavy snowfall over Montana ranges,
northern Idaho and along the Idaho/Wyoming border ranges to the
Park Range in northern Colorado and the northern Wasatch Utah.
Behind the cold front, bursts of snow will reach the valleys/high
Plains. Snow rates will reach 1-2"/hr which will accumulate rapid
as SLRs climb to as high as 15 to 18:1. Snow levels initially
around 1500-2000 ft will fall to ground level. Day 1 WPC
probabilities for more than 12 inches of snow are moderately-high
across the higher Northern Rockies with generally at least 3-6
inches over the surrounding valleys in MT.

As the main trough ejects activity southward Saturday and drives
the front into the Great Basin with associated height falls,
in-flowing Pacific moisture is shunted south of lift with more
dispersed heavy snow. Day 2 snow probabilities for over 6 inches
are moderate over the WA/OR Cascades and Blue/Salmon River Mtns as
well as WY to the Medicine Bow and Park ranges near the WY/CO
border. Pacific moisture refocuses ahead of the positively-tilted
trough over the Southwest with Day 3 snow probabilities for over 6
inches over the northern Sierra Nevada, eastern NV ranges, and
much of the UT Ranges with high probabilities for the Medicine
Bow/Park Ranges (where 3 day snow totals of 2-3 ft are forecast)
as well as the Laramie Range.


...North-Central Plains, Midwest, and Interior Northeast...
Days 1-3...

An impulse shifting southeast from the PacNW this morning will
track along the cold front banked against the eastern side of the
northern Rockies and eject over north-central Plains late this
afternoon, tracking near the SD/Neb border tonight and northern IA
Saturday morning/northern IL Saturday afternoon. As this impulse
moves into the High Plains, the interaction of these height
falls/PVA with the low-level baroclinic gradient and modest left
front diffluence ahead of an upper jet streak to drive surface
cyclogenesis in eastern WY this evening. This feature will remain
progressive as it moves eastward, but at the same time will deepen
across SD/NE thanks to the strengthening upper jet streak, before
shearing out into more confluent mid-level flow across the Midwest
Saturday night. Despite this fast evolution, downstream warm air
advection and the ageostrophic response to the strengthening upper
jet will result in a dynamic stripe of 850-600mb fgen to drive
intense omega into the column. A cross-hair signature with the
most intense UVVs intersecting the deepening DGZ as the column
cools along/behind the surface front, while the low/mid level WAA
will help deepen an isothermal layer beneath the DGZ to support
aggregate maintenance. Impressive fgen should produce a period of
heavy snow with max snow rates around 1"/hr per the 00Z HREF near
the SD/NE border tonight where WPC probabilities indicate 70% risk
for 6" or more of snow. With banding oriented with the direction
of motion, maxima over 8" are possible.

Overall forcing begins to wane as the shortwave shears into more
confluent flow to the east in response to height rises across the
Southeast still allows east-west banding with moderate rates north
of a surface trough over northern IA/IL into SW MI where Day 2
snow probabilities for 4"+ are moderate.

Cyclogenesis renews over the Northeast Sunday as the impulse
rounds the parent vortex center with wrap around snow over far
Upstate NY/far northern New England where Day 3 snow probabilities
for 4"+ are moderate north of the Adirondacks and along the New
England/Quebec border.


Significant icing is not forecast across the CONUS through Monday.
However, southern Plains/Mid-South icing will become a concern
Monday night.

Jackson